Processes for the preparation of reduced folates having an uniform (6S)- or (6R)-configuration are described in the French patent application No. 90 03032 having the publication-No. 2 659 330 (which corresponds to U.S. Pat. No. 5,239,074). Herein is also described the further prior art.
Herein below the abbreviation THF is used for 5,6,7,8-tetrahydrofolic acid.
It is known that the hydrolysis of N(5),N(10)-methenyl-THF.Cl.sup.- in an aqueous buffer at a neutral or basic pH-value leads to N(10)-formyl-THF; see D. R. Robinson, Methods in Enzymology (1971) Vol XVIII, Part B [184], and G. K. Smith, P. A. Benkovic, S. J. Benkovic, Biochem. (1981), 20, 4034, and J. C. Rabinowitz, Methods in Enzymology (1963), 6, 814.
It is also known that N(10)-formyl-THF can be transformed thermically in N(5)-formyl-THF; see D. R. Robinson, supra.
When a pure diastereoisomer of N(5),N(10)-methenyl-THF.Cl.sup.- is hydrolized, then the corresponding pure diastereoisomer of N(10)-formyl-THF is obtained; see G. K. Smith et al, supra.
It is now surprisingly found now quite surprisingly that the addition of an alkaline earth metal salt to an ammonium salt or an alkali metal salt of a mixture of diastereoisomers of (6RS)-N(10)-formyl-THF dissolved in water, or in an aqueous buffer solution, leads to the selective crystallization of the alkaline earth metal salts of (6R)-N(10)-formyl-THF.